Revelations of forced sterilisations among "undesirables" in Scandanavia and parts of Europe, some carried out as recently as the Seventies, have shocked Europe this week. But what lies behind them - and has attracted less attention - is Britain's role in the eugenics movement that swept Europe and North America in the Twenties and Thirties.

Ironically, Britain resisted moves to use selective breeding to improve the quality of the "human stock" despite harbouring some of the leading intellectuals responsible for popularising the theory. But Sweden, Norway, Denmark, Finland, Estonia, one Swiss canton and Nazi Germany all put the theory of enforced sterilisation into practice in the Twenties and Thirties.

Swedes have been shocked to discover this month that the policy was scrapped only in 1976, 31 years after the fall of the Third Reich. In the United States compulsory sterilisation laws were introduced in 30 states and were still valid in 19 in 1985.

Today such action is still supported in China where the premier, Li Peng, has declared that "idiots breed idiots" and a national eugenic law aimed at preventing "inferior births" came into effect in 1995.

The eugenics movement began, not with the imperialist ambitions of Nazi Germany, but with the recognition by a handful of scientists that some genes were better than others. It was backed by right and left alike and its history demonstrates the extraordinary reversals in moral thinking that occur over time.

The evolutionary theory of Charles Darwin, a president of the Eugenics Society, that only the fittest would survive became transmuted into a social philosophy that only the fittest should survive. It was supported by people as diverse as Winston Churchill, the Huxleys and early feminist birth controllers such as Marie Stopes who backed contraception because it both liberated women and provided a legitimate way of improving the gene pool.

In Sweden, grounds for enforced sterilisation included "unmistakable gypsy features, psychopathy, and vagabond life", according to one document. The principal grounds were "displaying undesirable racial characteristics" or signs of "inferiority," or "sexual or social deviancy."

It appears that the architects of Sweden's uniquely comprehensive welfare state, in a country characterised by strong social conformity, felt justified in preventing the birth of those who might make heavy demands on it.

The sexual paranoia which underlay eugenics received its clearest expression in Mein Kampf, in which Hitler's warning against the "blending of a higher with a lower race" was coloured by a violent fantasy: "With Satanic joy on his face, the black-haired Jewish youth lurks in wait for the unsuspecting girl whom he defiles with his blood ... with every means he tries to destroy the racial foundations of the people he has set out to subjugate."

The Nazi leadership took a more pragmatic approach, extending the principles of stud farming to the "Nordic" race. Walter Darre, head of the SS Race Office, wrote: "Just as we breed our Hanoverian horses using a few pure stallions and mares, so we will once again breed pure Nordic Germans."

Earlier in Britain, Francis Galton, who founded the science of eugenics, believed that citizens should be ranked in order of hereditary merit and the lower orders segregated in monasteries to prevent them procreating. Karl Pearson, a brilliant mathematician, claimed that a nation could not advance unless the "better stocks" flourish. He argued that Britain was declining because of the proliferating genes of the criminal, the sick and the mentally defective.

But Britain, despite providing the germ of these ideas, held out against acting on them. A Bill for compulsory sterilisation of certain categories of mental patient put before parliament in 1931 by a Labour MP, Major AG Church, was defeated.